Data communication of different kinds becomes more and more frequent today. This development demands higher bandwidth available for the communication. The capacity of different types of telecommunication systems has increased tremendously during the last decade. An increasing part of the capacity is supplied by optical fibers that present bandwidth enhancements of several magnitudes compared with ordinary wires.
The bandwidth of optical fibers is used in an efficient manner by employing Wavelength Division Multiplexing (WDM) techniques. Many channels using a multitude of slightly different wavelengths may be transported simultaneously in one and the same optical fiber without interfering with each other. In recent years Dense Wavelength Division Multiplexing (DWDM) techniques have developed, bringing the utilization of the frequency dimension one further step. The useful spectrum is divided in different bands, a S-band (Short band) ranging from 1460 nm to 1530 nm a C-band (Conventional band) ranging from 1525.6 nm to 1562.5 nm and a L-band (Long band) ranging from 1569.4 to 1612.8 nm.
A major problem in optical communication is the attenuation of optical signal due to inherent fiber losses. After being transported some distance, the optical signals are attenuated and have to be restored in one or another way. By introducing optical amplifiers, any transition into electronic signals is unnecessary. However, amplification of broad wavelength bands, e.g. the complete S-, C-, and L-bands, carrying a number of WDM channels is not completely straightforward. Several different amplifier approaches are presented in prior art.
Rare-earth doped optical fiber amplifiers are a class of optical amplifier widely used. They exhibit low noise, they can be operated over fairly large bandwidths and show negligible crosstalk. However, the operational wavelength region depends on the doping ion.
Optical amplifiers have also been based on Raman effects, through the Stimulated Raman Scattering (SRS). SRS is a nonlinear process in which new frequencies are generated through energy transfer between an optical wave and the medium, due to the excitation of an optical phonon. As it is a nonresonant process, gain is made available at any wavelength. In the case of silica, this frequency shift peaks around 13 THz from the pump frequency. The down-shifted frequency is known as the Stokes shifted frequency. The Raman gain extends over about 40 THz, but the useful bandwidth for application purposes is less than that.
A third type of optical amplifiers is a Fiber Optical Parametric Amplifier (FOPA). This type of amplifier has been studied intensively in recent years due to their potential use for amplification and wavelength conversion in Dense Wavelength Division Multiplexing (DWDM) transmission systems. They have attracted interest because the band of amplification depends on the design of the fiber used and thus can be moved outside the conventional rare-earth window band. This will allow the use of the full low-loss window of fused silica fiber. Fiber optical parametric amplifiers are able to operate in any of the telecommunication bands (S-C-L) depending upon pump wavelength and the fiber zero dispersion wavelength, which can in principle be appropriately tailored from 1300 nm to 1600 nm.
A fiber optical parametric amplifier operates based on the nonlinear process of wave mixing, whereby a pump source at a given wavelength close to the zero dispersion wavelength of an optical fiber leads to the generation of idler and signal bands from spontaneous noise. If an externally injected signal is simultaneously applied, it can be amplified in any of the signal or idler band, which are basically symmetrically located with respect to the pump wavelength.
Fiber optical parametric amplifiers are conventionally known for having a low efficiency, which means that very high laser pump power would be needed. The gain of a fiber optical parametric amplifier depends in general on three parameters; the nonlinear coefficient □, the length L of the fiber used as amplification medium and pump power PP. A low nonlinear coefficient calls for use of a high pump power or a long fiber length. However, recently, optical fibers having higher nonlinear coefficients have been even commercially available.
A relatively large problem with fiber optical parametric amplifiers is that the amplification principle gives rise to crosstalk. Optical signals having one wavelength will during the amplification process give rise to “false” signals at other wavelengths due to Four-Wave mixing (FWM). In DWDM systems, such crosstalk can generally not be accepted.